refactor: introduce internal command seam

[zatevakhin]

changes

• add the bounded application Command output subset and move semantic SessionListRequest into the application layer
• convert Command to ClientMsg only in protocol, preserving exact session-list, mesh, profile, session, input, and auth wire mappings
• migrate app, ACP state, mesh, and input outputs while adapting existing ClientMsg handler and event-loop boundaries with .into()
• make the compatibility ClientMsg session-list helpers delegate to Command constructors plus conversion as the single mapping source
• cover every command conversion, pending command draining for session updates and replays, and the event-loop LoadSession -> LoadSession + SubscribeSession send path

validation

• cargo fmt --all -- --check
• cargo check --all-targets
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --all-targets
• cargo test --all-targets --quiet (757 passed)
• architecture acceptance searches

follow-up debt

• the existing double-subscribe behavior is intentionally preserved in this seam PR
• pr3 runtime bus migration owns resolving load/subscribe ownership and removing the duplicate without mixing that behavior change into this PR

The temporary protocol conversion and SessionListRequest re-export are removed in the next stack item. The runtime bus remains ClientMsg in this PR.